Created Unassigned: Error with TabContainer V7.1213 [27644]

I have a set of tabs two of which start out hidden. When my code calls the 5th one it works OK. With it calls the 6th one I get a javascript error that shows up in IE. On other browsers the tab just comes up blank. I have no clue about all this. It was working until I upgraded to the latest version of the toolkit.

The c# is
TcBuls.ActiveTabIndex = 5;
TcBuls.ActiveTab.Visible = true;
which work OK.

TcBuls.ActiveTabIndex = 6;
TcBuls.ActiveTab.Visible = true;
which gives me this script error in jQuery.Tabs.TabContainer.debug.js

_setActiveTabIndex: function (value) {

var self = this, opt = self.options;

if (value < -1) {
throw "invalid tab index";
}

if (value >= self._tabs.length) {
value = self._tabs.length - 1;
}

if (!self._created || value != opt.activeTabIndex) {

if (opt.activeTabIndex != -1) {
self._tabs[opt.activeTabIndex]._set_active(false);
}

Line 172:
{exception} Unable to get property '_set_active' of undefined or null reference